Spinal implants can be engaged to or along one or more vertebrae of the spinal column for the treatment of various spinal conditions. For example, elongated connecting elements, such as rods, plates, tethers, wires, cables, and other devices have been implanted along the spinal column and connected between two or more anchors engaged between one or more spinal motion segments. Fasteners can be provided to secure the implant to a particular location along the spinal column. The implants can be provided to stabilize the spinal column for treatment, either by fixing the spinal column or by permitting at least some motion of the stabilized motion segments.
Anchor assemblies such as multi-axial and uni-axial screws have been employed for securing elongated implants, such as rods or plates, along one or more motion segments of the spinal column. Such screws can comprise many components or parts to secure the implant to the screw, and such components can interact with one another and with the screw shaft to allow angular movement relative to the shaft for orienting the assembly into proper alignment with the elongate implant, and then cooperate with one another to rigidly lock the elongate implant into a fixed position relative to the screw shaft. The interaction between the components of these screws and the elongated implant and the complex spinal anatomy can result in less than optimal outcomes, some of which relate to stresses that are placed on the anchor after implantation, for example, resulting from normal activity of the patient. Dynamic anchor assemblies that allow movement of assembly components coupled to elongate members relative to assembly components coupled to bony structures of the patient after locking the assembly to elongate members are needed.